What is the purpose of the "active" toggle on fcurve modifiers?
For example, if I click on the noise modifier and it put my mouse over it, it says "F-curve modifier is the one being edited":


and if I click on the noise modifier and it put my mouse over it, it says "F-curve modifier is the one being edited":


The graph displayed looks the same whether one or the other is selected.

It basically says which modifier you clicked on

Maybe this could help:
http://www.blender.org/api/blender_python_api_2_75_3/bpy.types.FModifier.html